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Glossary of Terms

The Skeletal Animation System (SAS) uses terms which you probably don't use in everyday conversation. You may want to refer to this chapter if you're confused about a term when using the other chapters in this book or the Skeletal Animation System Tutorial.

Skeletal Animation System-A Glossary

For example, Figure 1.2 shows the same skeleton reaching for the same location using different IK Sets.

Figure 1.2 Executing an IK move with different IK Sets

You must define an IK set before performing any IK move-if you attempt to do so, you're prompted to define the IK set before you continue. IK Sets are created and modified using a simple matrix, such as the one shown in Figure 1.3:

Figure 1.3 IK Set

Hard Part

The default skin part type for skins assigned to a skeleton using the Skin operation. Parts can be defined as "hard" or "soft" using the Hard/Soft command. Vertices in a hard part retain their relationship to a bone, regardless of how that bone is moved; vertices assigned to soft parts deform naturally around joints on a skeleton. Hard and soft parts are described in more detail in the Skeletal Animation System Tutorial. See also soft parts.

Inverse Kinematics

Inverse kinematics is an algorithm used to pose skeletons. Inverse kinematics says that if you change a joint's orientation, the rest of the skeleton can be made to behave in a natural way to accommodate the new position of that joint.

Inverse kinematic moves pays attention to any DOF limits placed on bones affected by that joint's positional change.

When you perform an IK move, you can move the point within defined constraints; in fact, IK constraints in place for that joint may make certain positions unattainable for the skeleton. Defining those constraints more exactly ensures more expected skeletal motion (as there are fewer possible positions for affected bones to take to accommodate the point move).

Joint

Each bone is defined by two joints, one at each end. The joint further from the root is referred to as the inferior joint of the bone (the pointy end of the bone frame), the joint closer to the root as the superior (the wide end of the bone frame).

Figure 1.1 Head and tail joints

Joints can be assigned a specific joint ID using the Joint Identification command, which automatically defines DOF limits to be used when performing IK operations on this joint.

Local Axes

Each bone has local X, Y, and Z axes. Typically, one of these axes is aligned with the length of the bone; this axis is referred to as the bone direction axis or twist axis.

Figure 1.2 Local bone axes

When you modify the position of a bone, having the local axes follow the orientation of the bone is optional. However, the local axes for a bone always change if the bone's superior moves.

For example, in a humanoid skeleton, if you move the humerus (upper arm) bone, the xyz axes for the forearm, wrist, and any bones in the hand are updated automatically because the humerus is superior to those bones.

Motion Capture Data

A skeleton can have motion data associated with it. Motion capture data causes the skeleton to move in a specified manner over a period of time using N-Dynamics. Motion capture data is generated based on computer input from motion sensors attached to live models. Motion capture data is supplied by other third party vendors, not by Nichimen Graphics.

N-Geometry currently supports three formats of motion capture data:

.asf (Acclaim format)

.bvh (BioVision format)

htr (Motion Analysis)

These are all editable ASCII files.

Each of these types of motion capture data can be decomposed into component data:

rotational components (shows the x, y, and z rotations for each bone over the length of the file)

pose to pose (allows you to capture poses based on the motion of the skeleton)

Both of these techniques are described in the Skeletal Animation System Tutorial.

Pose

You can pose a skeleton in different positions (much like a wire model), then save those poses for use later with N-Dynamics. Poses are measured in relation to the saved base state for the skeleton (or relative to another pose with difference poses). Poses are the sum of any local bone rotations plus any root transformations.

Root

The root is the "logical" center of the skeleton; any bone can be traced along a single path back to the root.

: Note. No loops are allowed in a skeleton. There is only one path back to the root from any joint.

Typically, the root also corresponds to the center of gravity for the skeleton. For a humanoid, the root is usually at the base of the spine:

Figure 1.3 The root for a humanoid skeleton (Biovision)

: Note. When scaling a skeleton, you should always scale by selected vertex, choosing the root as the vertex around which the skeleton is to be scaled. This ensures that the root remains aligned with the global origin.

Skeleton

The skeleton is a special type of geometry body which can be created in or imported into N-Geometry. By attaching models (skins) either to the entire skeleton or to individual bones on the skeleton, you can animate the transformation and deformation of those objects. What other systems use a hierarchy of bones to represent is incorporated into a single skeleton body.

Skin

The skin is the 3D object or model that you want to actually animate. When assigning a 3D "hand" to the "hand bone," the hand is referred to as the skin. Skins act like "envelopes" around the skeleton which deform to follow the animated skeleton. See also attached object and parts.

Figure 1.4 The skin and skeleton

Skin Displacements

Skin displacements are "secondary" actions that can be created by the animator to cause the change in skin shape based on skeletal animation to more closely simulate natural movement. Skin displacements are similar to relative displacements in N-Geometry. They can be used to simulate muscular expansion or facial expressions.

For example, a forearm rotating upward in the X direction should display a corresponding swelling of the bicep and change in shape of the elbow joint. The swelling of the bicep and the change in shape of the elbow can be saved as a skin displacement; when animating the skeleton (rotating the forearm upward in the X, for example) the skin will then change shape appropriately every time.

Skin Parts

When a skin is attached to a skeleton, it is divided into "parts." Each set of vertices (part) is associated with a bone on the skeleton, and are driven by the animation of that bone.

Soft Influence

The calculation method used to determine how soft parts are deformed; see also hard parts, soft parts.

Soft Parts

Individual skin parts can be defined as "hard" or "soft" parts; soft parts allow for natural deformation around areas like joints on an animated figure. When a part is designated as soft, additional calculations are performed during skeletal animation to make the appearance of the skin around the joint more natural. See also hard parts.

Superior and Inferior

If a bone (B) lies between a selected bone (C) and the root (A), it is said to be superior to that bone. If a bone is further away from the root, it is inferior to that bone. A bone may have only one path to the root (see Root above).

For example, the bone in the upper arm is superior to the bone in the forearm, since it lies between the forearm and the root. Consequently, the forearm bone is said to be inferior to the upper arm bone.

Figure 1.5 The upper arm is superior to the forearm; it is between the forearm and the root

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Glossary of Terms

Skeletal Animation System-A Glossary

Attached Objects